1. Field of the Invention
This invention generally relates to a water insoluble additive for improving the performance of an ion exchange membrane, particularly in the context of high temperature operation of electrochemical fuel cells.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, a cathode and an anode. An electrocatalyst induces the desired electrochemical reactions at the electrodes. In addition to the electrocatalyst, the electrodes may also contain an electrically conductive substrate upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black (such as a substantially pure, unsupported finely divided metal or metal powder) an alloy, or a supported metal catalyst (such as platinum on carbon particles).
One type of electrochemical fuel cell is a proton exchange membrane (PEM) fuel cell. Such fuel cells employ a membrane electrode assembly (MEA) comprising an ion-exchange membrane as the electrolyte disposed between the two electrodes. Ion-exchange membranes that have received considerable attention are those prepared from fluoropolymers and which contain pendant sulfonic acid functional groups functional groups. A representative polymer in this regard can be obtained from DuPont Inc. under the trade name Nafion®.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute oxygen stream such as air.
The electrochemical oxidation that occurs at the anode electrocatalyst of a PEM fuel cell results in the generation of cationic species, typically protons. These protons must then cross the electrolyte to the cathode electrocatalyst where reaction with the oxidant generates water, thereby completing the electrochemistry. Typically, transport of protons across the ion-exchange membrane is assisted by water molecules. Thus, humidification of the ion-exchange membrane has been found to improve conductivity and hence fuel cell performance. In the case of Nafion®, high conductivity is observed in the presence of water due to the movement of protons between sulfonate clusters. In the absence of water, such free movement of protons is restricted and conductivity of the electrolyte is significantly decreased.
Traditionally, operation of PEM fuel cells have been limited to operational temperatures below 100° C. to limit dehydration of the ion-exchange membrane. At temperatures above 100° C., the vapor pressure of water increases rapidly, resulting in dehydration of the ion-exchange membrane and operational difficulties. For example, one technique for operating electrochemical fuel cells at temperature above 100° C. is to employ a pressurized humidification system for maintaining hydration of the electrolyte. Other techniques have involved attempts to improve fuel cell performance under low humidity conditions (which provides benefits at operational temperatures both above and below 100° C.).
One technique for improving fuel cell performance under low humidity conditions involves acid doping of the ion-exchange membrane with, for example, phosphoric acid. Such acid molecules act as the proton-conducting medium and are held in the membrane by non-covalent, acid-base ionic interactions. For example, phosphoric acid doping of polybenzimidazole (PBI) resin has shown some promise as an electrolyte for high temperature fuel cells. The phosphoric acid molecules are associated with the basic imidazole nitrogen atom through hydrogen bonding (see Wainright et al., J. Electrochem. Soc. 142(7):L121-123, 1995; U.S. Pat. No. 5,525,436). However, for such compositions, the operational temperature of the fuel cell must be maintained above 100° C. If the fuel cell falls below this temperature, condensed water within the fuel cell washes out the acid molecules, thus resulting in decreased performance (see, e.g., U.S. Published Application No. US2002/0068207).
The limitations associated with prior acid doping techniques have lead to further research in this area in an effort to better retain the acid molecules within the acid-doped membrane. For example, one technique involves doping of phosphoric acid molecules into a porous polybenzimidazole (PBI) membrane prepared through coagulation with subsequent drying, and then collapsing the membrane to physically trap the acid molecules (see U.S. Pat. Nos. 5,599,639 and 6,187,231). Another technique involves soaking finally divided PBI polymer in an acid that result in dissolution of the polymer and formation of a paste or gel that can then be applied to a polymer fabric or used directly as the electrolyte in a fuel cell (U.S. Pat. No. 5,945,233). While these techniques report improvements in retention of the doped acid, the amount of bound acid molecules per monomer repeat unit of the polymer does not change, and leaching of the unbound acid inevitably results in a decrease in performance of the fuel cell.
To reduce leaching of acid-doped membranes, attempts have been made to dope with organic sulfonic or phosphoric acids (see U.S. Pat. No. 6,124,060), or by covalently bonding organic sulfonic or phosphoric acids through N-alkyl or N-aryl linkages (see U.S. Pat. No. 4,933,397). Similarly, U.S. Pat. No. 4,634,530 is directed to the formation of a covalently bonded sulfonated PBI membrane by contacting the PBI membrane with a sulfonating agent, followed by heating the same for a period of time sufficient to convert the ionic bonds formed in the contacting step to covalent bonds.
More recently, water insoluble additives have been developed comprising a metal oxide cross-linked matrix having phosphonic acid groups covalently attached to the matrix through linkers (see Published U.S. Application No. US 2005/0112439 A1).
While advances have been made in this field, there remains a need for improved ion-exchange membranes for use in high temperature fuel cells. Furthermore, during start up, shut down, or low load conditions, such high temperature fuel cells may operate for some period of time at lower temperatures, such as lower than 100° C. Thus, improved ion-exchange membranes are needed that will perform to acceptable degrees over a wide range of operational temperatures. The present invention fulfils these needs and provides further advantages.